1. Field of the Invention
The present invention generally relates to lithography, and more particularly to systems and methods for focusing a lithographic tool.
2. Background Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.
The substrate is imaged using an exposure system. Before the substrate is imaged, however, a focus sensor maps the topography of the top surface of the substrate to determine focus-positioning parameters of the exposure system. Ideally, the focus sensor should sense the top surface of the substrate, and not be affected by underlying layers deposited on the substrate as part of the fabrication process. Additionally, the focus sensor should (i) comply with mechanical packaging constraints, (ii) have a high bandwidth (in order to comply with a desired throughput of the lithographic apparatus), and (iii) have a reasonable working gap (in order to protect the substrate (e.g., silicon wafer)). Commonly used focus sensors include optical and capacitance sensor systems.
Unfortunately, optical and capacitance sensor systems are often affected by the optical and capacitive properties of the underlying layers. For example, light used in an optical sensor system may reflect off an underlying layer and interfere with light reflected off the top surface. Similarly, capacitance sensor systems can be affected, for example, by the permeability and conductivity of the deposited layers, as well as quality and thickness of polish and oxide on the bottom surface of the substrate.
Because optical and capacitance sensor systems may interact with the underlying layers, these types of sensor systems may misread the top surface of the substrate. Errors that are dependent on layers deposited as a result of chip production are called process errors. As a result, data obtained from the optical and capacitance sensor systems will often include these process errors. In fact, the process errors of an optical sensor system can be even larger than the physical thickness of the deposited layers on the substrate. Significant work is then needed to attempt to reduce the process errors, and some focus accuracy may even be lost.